From WO 98/31963 a swivel is known having a toroïdal chamber for receiving hydrocarbons. The toroïdal chamber is enclosed by a stationary inner wall and a rotary outer wall. The stationary inner wall is provided with a first guide for supplying hydrocarbons from a fluid line connected to the seabed towards the toroïdal chamber. The rotary outer wall is provided with a further guide to allow the hydrocarbons to be forwarded towards product piping on board of a vessel.
In order to define the toroïdal chamber between the two walls, the inner wall and the outer wall are located with their adjacent cylindrical surfaces in close proximity, whereby a relatively small upper annular gap and a lower annular gap are formed between the adjacent cylindrical surfaces of the walls on either side of the toroïdal chamber. In the art the gaps are also known as ‘seal extrusion gaps’.
In order to avoid any leaking, these gaps are sealed by means of sealing elements. These sealing elements normally comprise elastically deformable sealing rings which are of U-shaped or V-shaped form.
The sealing of swivels handling liquids or crude oils with limited gas content can also be achieved with a double sealing arrangement on either side of the toroïdal chamber. The first or primary seals are positioned closest to the toroïdal chamber and carry full pressure. The second or secondary seals are positioned at a distant of the first or primary seals and function to provide back-up in the event the first or primary seal should rupture.
In the known swivels the gaps are designed to have a minimal opening. However, a minimal gap width must be maintained such that possible deflections of the inner and the outer walls of the swivel will not cause the two elements to contact one another and thus prevent relative rotation of the two swivel elements.
Normally the swivel, according to the type above, will be part of a swivel stack positioned in a turret. The subsea hydrocarbons structures, such as oil or gas wells, are connected to a floating production, storage and offload vessel (FPSO) via one or more risers extending from the bottom of the sea to the vessel. By means of the swivels, the risers are each connected to a piping structure on a turret around which the production vessel can weathervane to adjust its position according to current directions and prevailing wind. The geo-stationary part of each product riser is connected to a stationary inner ring of a respective swivel in the swivel stack. Production piping for distributing the hydrocarbons to the weathervaning part of the vessel are connected to the outer annual ring of each swivel which can rotate with the weathervaning vessel around the fixed positioned inner ring of the swivel.
In order to minimize the possible deformations of the inner and the outer wall due to pressure effects, the known swivel walls are made of cast steel with a wall thickness up to 240 mm for a swivel with a diameter of 2 m. Deformations are so prevented by the heavy masses of the inner and outer wall. The effect of these measures is that a swivel with a 2 m diameter may weigh 20 tons or more.
When designing a swivel, not only the deformations of the inner and the outer wall should be minimized, the design should also ensure that the stresses remain within the limits imposed by the Pressure Vessel Codes.
In use the swivels and specially the outer part of the swivels are exposed to heavy load and important temperature changes.
Hydrocarbons supplied to the swivels often have temperatures in the order of 20 to 120° C. During the transfer of the fluids the outer wall will be cooled off by the external ambient air, especially when the swivel is exposed to outside weather conditions and when the wind directly blows against the exterior of the swivel. Contrary to the outer wall, the inner wall will remain relatively hot.
The temperature difference between the inner wall and the outer wall can also be important when the fluid transferred via the swivel is very cold; this is the case for swivels allowing the transfer of liquefied gases such as LPG, LNG, CO2 and any other cold or cryogenic fluids.
Generally, the swivels are exposed to high pressure. For the purpose of the present invention the word high pressure is intended to include pressures of 50 by and higher. The pressure in the toroïdal chamber of the swivel can easily exceed pressures of 200 bar.
Because of the extreme conditions under which the swivels are used, it is possible that during the lifetime of the swivel the outer annular ring presents defects and must be replaced. Because of the fact that each swivel is part of a swivel stack, which is positioned in a turret, the removal and replacement of an outer annular ring in a swivel is extremely complicated. Because of the presence of adjacent swivels there is very little room to move the outer annular ring of a swivel either upwardly or downwardly. In order to replace the outer annular ring of a swivel, the swivel has to be removed from the swivel stack completely. Once the outer annular ring has been replaced, the swivel can be positioned in the swivel stack and be used for its normal function.
According to the prior art, the replacement of an outer annular ring in a swivel stack is either done by using a barge next to the turret, which barge is provided with lifting means in order to remove the different parts of the swivel stack in order to create enough accessibility to replace the outer annual ring which has reached the end of its lifetime. Alternatively, the vessel should be completely cut loose from the production lines and should be sailed to a workshop, for instance on shore, in order to do the replacement of the swivel part on shore.
In practice, the removal of the swivel stack and replacement of an outer annular ring of a swivel can take several months. In the offshore technology, the downtime of any system is extremely costly. Because of the costs of intensive equipment that is used and the costs of personnel any downtime should be limited to the absolute minimum.
In order to avoid at least some of the problems described above, related to the replacement of a swivel part, the object of the present invention is to provide a method for replacing an outer annular ring of a swivel, which method is both time and cost efficient.